The invention relates to a process and apparatus to establish contact between liquid and vapour or gas, particularly on trays used in distillation and/or absorption columns.
As known, in order to establish close contact between the ascending vapour or gas and the downflowing liquid on the active surface of trays used in columns for carrying out distillation and absorption, generally the same elements (e.g. bells, valves, or holes) are used, the individual dimensions of which are identical on the whole active surface of tray. The "active surface" of the tray is the area of the tray reduced by the inlet and downflow areas. Some tray constructions are also known, wherein several types of the mentioned contacting elements are used simultaneously. Such is for example the sieve- or valve-type tray, which--beside the valve on the tray--is provided with separate holes.
This group includes the so-called "co-current" trays, on the active surface of which such elements are evenly distributed, or used at increasing rate in the main flow direction of the liquid, where the exit velocity or impulse of the vapour or gas flow has a horizontal component in the flow direction of the liquid. The flow direction of the vapour or gas immediately after exit from the contacting elements on these trays is the same as the main direction of the flow rate, hence the vapour or gas flow does not induce recirculation of the liquid, in contrast with the bell-, or valve-type trays. Such co-current tray is described in the DE-PS 1 632 219. The tray is provided with tongues compressed from the sheet metal of the tray. According to the DE-PS 3 417 975, the co-current slotted sieve tray is provided with orifices compressed also from the sheet metal, and arranged between the holes of the traditional sieve-tray. The special contacting elements of both trays protrude from the plane of the tray and thus they baffle the flow rate. The circular segment shaped areas are generally adjacent to the active surface of the trays serving for the inlet and outlet of the flow rate. Downflow weir can be arranged at the boundary of the active tray-surface and the down flow area of the liquid. The stagnant zones--dead spots--represent drawback for all the known tray constructions, where no contact exists between the two phases (liquid and vapour or gas). These dead spots cannot be eliminated for constructional reasons. All above described tray constructions have only a single function: namely bubbling of the ascending vapour or gas through the liquid layer conducted generally in cross-flow on the tray.
It is known however, that the compositions of the vapour or gas ascending from the trays of the distillation and/or absorption columns, as well as those of the liquid leaving the trays are not in thermodynamic balance, in other words the trays of known construction are unbalanced, i.e. their efficiency is lower than that of a theoretical tray. The reason for this--according to the generally accepted molecular diffusion theory--is that the balance between the liquid and vapour or gas would require infinitely long contact time and infinitely large contact surface. At the same time, the vapour or gas flow should perfectly be mixed with the liquid on the tray. The presently known trays are not suitable to satisfy these requirements and their efficiency, i.e. separating capacity is reduced by the stagnant zones (dead spots), thus it is automatically lower than the theoretically possible maximum capacity. Similar problems crop up in other cases of contacting gas and liquid, e.g. in the aeration of waste water, or other water to be treated.
Object of the present invention is to provide a process and apparatus for contacting vapour or gas and liquid, where the contact is more efficient than that of the earlier solutions, and results in a balanced or nearly balanced operation of the trays in columns used for carrying out distillation and/or absorption, and thus in maximal efficiency in contacting the phases.
It has been found that if the velocity of the flow rate conducted on to the tray is increased to a value exceeding the velocity of the vapour or gas bubbles ascending from the liquid upon the buoyant effect, while a turbulent liquid flow of lower velocity than this limit velocity is produced in the domain of the tray before the downflow area, then the balanced or nearly balanced operation of the tray, and thus the maximal efficiency of the contact between the liquid and vapour or gas can be accomplished. Namely in the first phase--accelerating zone--the liquid flowing at high velocity in thin (a few mm) layers, partly entrains the bubbles arising from the vapour or gas admitted from below, and partly the bubbles are comminuted by the arising unidirectional, high shear stress. No bubble ascent is experienced in this accelerating zone. In the second phase--degasifying zone--partly the vapour or gas bubbles entrained and conducted by the liquid are liberated, and leave the liquid mass, and partly the bubbles entering this zone from below will also move off.
In the whole area of the degasifying zone such intensive turbulence will develop, that complete mixing, free from dead spots will come about. Since the perfect mixing of the vapour or gas and liquid is conditional upon attaining the theoretical, balanced tray, thus the degasifying zone is equivalent to a theoretical, i.e. balanced tray.